The project continues to be devoted to the development of various test systems suitable for testing drugs potentially effective in cancer of the prostate. In the past, several in vivo and in vitro systems utilizing the prostates of dogs, rats and baboons have been utilized in which the activity of 5alpha-reductase and arginase, deposition of androgens and estrogens and the uptake of labeled zinc were used as indices for the effectiveness of a range of chemotherapeutic agents. Some of the effective agents have been used successfully in the treatment of cancer of the prostate, whereas others, which were effective in animal systems, proved to be too toxic and/or ineffective in human prostatic cancer. Hence, more recently, our major efforts have been and continue to be directed towards the utilization of organ culture of human cancerous and non-cancerous prostatic tissue as a means of testing the effectiveness of various antitumor agents. The indices used include histology, growth rate and a number of biochemical parameters, particularly 5alpha-reductase activity, labeled zinc uptake and the metabolism of androgens. This approach should afford a more direct evaluation of the effectiveness of drugs on human prostatic tumors and may prove to be a more effective and appropriate system than the animal in vivo models.